It is known that the power output of combustion turbines and associated power generators is dependent upon the ambient temperature of the air as well as its pressure. Reduced pressure and increased ambient air temperature have the effect of reducing mass flow of gas through the combustion turbine resulting in decreased power output for the combustion turbine driving the generator. The power output of a combustion turbine (CT) may be enhanced by various methods including inlet air humidification, inlet air cooling, and injection of superheated steam or superheated humidified air into the combustor of the combustion turbine. Generally these techniques increase the mass flow of gas through the combustion turbine to offset low air density due to an increased ambient air temperature or reduced pressure for high elevation installations.
Steam that is injected into a combustion turbine must be of sufficient purity so that the blades of the turbine will not be damaged. The major concern is nonvolatile or condensable matter, such as entrained solid particles or dissolved material that may be deposited on the turbine blades or could melt in the combustor and deposit on the turbine blades. When steam or humidified air is injected into the combustor of a combustion turbine to enhance power output, it must have a very low entrained solids content. The specific limit for solids content depends upon the turbine design, the purity of the intake air, and the purity of the fuel, among other variables. A typical solids concentration limit for a combustion turbine with humidified air injection (CT-HAI) is 0.5 ppm solids by mass in the injection stream.
The common method for generating injection-quality steam for a CT without an existing heat recovery steam generation (HRSG) is to use demineralized water and a once-through boiler/superheater. Due to the capital cost of a demineralized water system and its associated operating cost, steam generated in this manner has been used more typically for emissions control than for power augmentation. For CT-HAI, a requirement of demineralized water adversely affects project economics.
Humidified air may be added to a combustion turbine using demineralized water fed into a once-through boiler/superheater to produce superheated steam for mixing with compressed air and subsequent injection into the combustor of the combustion turbine. The boiler/superheater may utilize hot exhaust gas from the turbine, a heat recovery unit, to superheat the water. A schematic diagram of a CT-HAI cycle using demineralized boiler feed water (BFW) is shown in FIG. 3. In this system, all demineralized water fed into the once through boiler/superheater is converted directly into superheated steam, mixed with compressed air, and fed to the turbine. The steam is only as pure as the demineralized feed water from which it is formed. This requires a high degree of demineralization, and a lack of chemical additives and corrosion inhibitors in the water requires the boiler tubes to be of costly alloy construction.
To avoid the economic disadvantages of demineralized water when using the large water flows required for power augmentation, the saturator concept was developed. While it is a suitable technical solution, it requires a costly saturator vessel and high-alloy water heater tubes, while adding system complexity. Rather than once-through boiling and superheating the steam, humidified air may be added to a combustion turbine using a saturator. A combustion turbine humidified air injection (CT-HAI) system with a saturator is shown in FIG. 4. Such a saturator system may be used in a combustion turbine humidified air injection system with softened, potable feed water. In the saturator system, potable aerated water is heated by re-circulation through a loop including a water heater coil, a pump, and a saturator. Compressed air is fed into the saturator and as the water is contacted with air in the saturator, the water evaporates to form humidified air. As water is evaporated, the dissolved solids concentration in the unvaporized, recirculating liquid water will increase. The concentration of total dissolved solids (TDS) is limited to acceptable levels in this unevaporated water by continuous or periodic blowdown of water from the saturator, typically about 20% of the water feed rate. Make-up water is fed to the system to balance the outflow of water in the humidified air and the blowdown streams. The humidified air leaving the saturator will carry off small droplets of water containing dissolved solids, but the mass flow of droplets exiting the saturator will usually be on the order of 0.05% of the humidified air flow. If the blowdown rate is adjusted to produce a total dissolved solids (TDS) concentration of, say, 1000 ppm in the saturator, then the solids carried out by the droplets in the humidified air will amount to 0.5 ppm by mass in the humidified air stream, which is sufficiently pure for almost all combustion turbine injection applications. The humidified air leaving the saturator is subsequently superheated before being injected into the combustion turbine CT combustor. The superheater may utilize hot exhaust gas from the turbine to superheat the water.
The heating of aerated water to the range of about 350° F. to about 500° F. as is typical for the water heater portion of the saturator requires an oxidation-resistant alloy for heat exchanger or water heater tubes. Austenitic stainless steels (304, 316, etc.) have acceptable oxidation resistance, but they are not suitable for use in the water heater because of their lack of resistance to chloride stress corrosion cracking (SCC), which particularly affects austenitic stainless steels. Duplex stainless steels such as Alloy 2205 have reasonable resistance to chloride stress corrosion cracking, but duplex alloys cannot be used for the water heater tubes because of embrittlement problems if exposed to temperatures above about 885° F. (474° C.). The American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code does not permit the use of ferritic stainless steels or duplex stainless steels for design temperatures above 600° F. (315.5° C.) because of this reason. Since the water heater or heat exchanger of a combustion turbine with humidified air injection (CT-HAI) is not always in service, the tubes in such a heat exchanger could be heated by the hot turbine exhaust to temperatures exceeding 1100° F. (593° C.). The requirements for oxidation resistance, chloride stress corrosion cracking (“SCC”) resistance, and no irreversible deterioration of physical properties at temperatures exceeding 1100° F. (593° C.) generally require the use of a costly high-nickel alloy, such as Alloy 625 or Alloy G, for the water heater tubes.
There exists a need for an apparatus and method of making superheated steam, or steam-air mixtures, for combustion turbines that does not require expensive alloys for construction of heat exchangers, demineralizers for purifying water, or expensive pumps for re-circulating water in saturator feed loops.